Re: [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz" again

[Ingo Molnar]

* Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> wrote:

> Hi Linus,
> 
> On 11/9/2017 11:38 AM, WANG Chao wrote:
> > Commit 941f5f0f6ef5 (x86: CPU: Fix up "cpu MHz" in /proc/cpuinfo) caused
> > a serious performance issue when reading from /proc/cpuinfo on system
> > with aperfmperf.
> > 
> > For each cpu, arch_freq_get_on_cpu() sleeps 20ms to get its frequency.
> > On a system with 64 cpus, it takes 1.5s to finish running `cat
> > /proc/cpuinfo`, while it previously was done in 15ms.
> 
> Honestly, I'm not sure what to do to address this ATM.
> 
> The last requested frequency is only available in the non-HWP case, so it
> cannot be used universally.

This is a serious regression that needs to be fixed ASAP, because the slowdown is 
utterly ridiculous on a 120 CPU system:

  fomalhaut:~> time cat /proc/cpuinfo  >/dev/null

  real    0m2.689s
  user    0m0.001s
  sys     0m0.007s

Thanks,

	Ingo